Sheets of transparent material carrying written or diagrammatic information are commonly referred to as transparencies. They are widely used in education for projection through an overhead projector either singly or superimposed. They are also used in direct demonstrations without a projector for superimposing added features to opaque background sheets.
The usual method of ensuring the accurate positioning of the superimposed features is to hinge the overlay sheets to the background sheet like the pages of a book.
Hinging in this way is not suitable for many applications where it is necessary to demonstrate that a single superimposed feature is capable of fitting at two distinct positions on the background. For these applications the overlay sheet is not fixed to the background sheet in any way. The demonstration is achieved by sliding the overlay sheet over the background sheet, freehand and stopping at the two correct positions. This freehand operation is very awkward to perform accurately.